Nuclei – Formation of Elements and Fundamental Properties

Chart of nuclides with the colour code showing the binding energy per nucleon: the most stable nuclides around iron are in dark blue.

The chemical composition of our universe shows some surprising peculiarities: The sun mainly consists of hydrogen and helium; iron is much more abundant on Earth compared to heavy elements like gold. Nucleosynthesis follows reaction paths involving fusion and capture processes, some of them yet mostly unexplained. Since nuclear fusion stops at iron, heavier elements are generated via proton or neutron capture under extreme conditions like in supernova explosions of stars or in hot environments like accretion discs around Black Holes or neutron stars.

Based on Einstein’s principle of mass-energy equivalence, high-precision mass measurements are used to determine nuclear binding energies which are crucial for reaction pathways in nucleosynthesis. The direct determination of superheavy nuclear masses bridges the gap to the island of stability predicted by benchmarking theories. In analogy to the electrons in the atomic shell, the nuclear structure is described by a shell model. Hitherto unknown shell closures and nuclei with “magic” numbers of nucleons far from stability are searched for by means of highly precise mass measurements on mostly short-lived exotic nuclei, first of all such possessing a large excess of neutrons, as they occur also in neutron stars.

Traps and Cryogenic Storage Ring

The PENTATRAP Penning trap for highly charged ions.

Ions can be stored in traps by the superposition of electric and magnetic fields in an extreme vacuum. Penning traps allow storage of a single ion that performs a characteristic oscillating circular motion in the trap. The ion’s mass and further properties can be deduced from the frequency if the charge state and the magnetic field strength are known, even in the case of exotic particles that live only for a few milliseconds. Penning-trap mass spectrometers are operated at MPIK and at radioactive beam facilities like GSI and CERN.

In an electron-beam ion trap (EBIT), highly charged ions are produced by impact of energetic electrons, then spatially confined, and electronically heated up to temperatures of millions of degrees. Both, stationary and mobile EBITs are used to prepare and study atomic matter under extreme conditions. A suite of accurate spectroscopic instrumentation attached to the EBITs collects precise data. A new cryogenic ion trap (Cryogenic Paul Trap Experiment: CryPTEx) has been built at MPIK in cooperation with the university of Aarhus, in which ion crystals can be produced by means of laser cooling and highly charged ions cooled therein.

In the electrostatic cryogenic storage ring, CSR, cold molecular ions of any size and highly charged ions can be investigated for the very first time essentially without any influence of the environment. This is achieved by a purely electrostatic ion optics, keeping the ring under extremely low pressure and a temperature of a few degrees above absolute zero. The ions are produced in dedicated ion sources and injected into the ring by high voltages of up to 300 kV. In addition, a device for injecting beams of neutral atoms is attached to the CSR. An electron cooler will narrow the stored ion beam. The innovative mechanical concept of the CSR has been developed and realized in close cooperation with MPIK’s engineering design office and precision mechanics shop.

Laboratory Astrophysics – the Chemistry of Space

The puzzle of cosmic chemistry in interstellar clouds.

One puzzling question is the formation of organic compounds in interstellar clouds. This complex chemistry is driven by reactions with ions and radicals which are created in collisions with photons and cold electrons. Here, the H3+ molecule plays a key role. The break-up of molecules after capture of an electron (“dissociative recombination”) can be studied in detail in storage rings. With the new cryogenic storage ring CSR, for the first time conditions are reached that correspond to space temperatures where also the rotations of molecular ions are in fact “frozen”.

First studies at temperatures below 15 K could already be performed with the CSR prototype, the linear ion trap CTF. Negatively charged molecular ions (anions) are here of particular interest as they represent an important source of slow electrons. Provided sufficient inner excitation (vibration), they can literally “evaporate” electrons. Collisions with neutral atoms and molecules are also of great importance for astrochemistry. A new collision section for neutral beams at the CSR will provide access to this experimentally yet largely unexplored field.

Highly Charged Ions – Matter under Extreme Conditions

Spectrum of iron ions which determine the radiation transport within the Sun.

Highly charged ions are found in hot environments of more than one million degrees such as stellar atmospheres and cores, supernova remnants or accretion discs around neutron stars and black holes. In fact, most of the visible matter in the Universe is assumed to be highly ionized. Analysis of the observed light (visible, UV or X-ray) from these ions needs support by theoretical structure calculations which are often not accurate enough to determine e. g. the temperature of the hot environment. The controlled production of highly charged ions in an EBIT combined with high-precision spectroscopy provides direct experimental information. One example is the investigation of the X-ray absorption of highly charged iron ions at the synchrotron PETRA III (DESY) which provided important new insight into the radiation transport in stars.

The cryogenic ion trap CryPTEx provides efficient cooling of trapped HCIs for high-precision laser spectroscopy. In collaboration with the PTB (Braunschweig), the MPIK contributes to the development of novel optical clocks using quantum logic spectroscopy. The ultimate goal will be to test the time dependence of natural constants.